1) Field Of The Invention
The present invention relates to a sheet-like structure consisting of a substrate and a coating on at least one surface of the substrate, and a process for its production. The coating is applied as an aerosol simultaneously with a corona treatment process to the substrate. The aerosol contains a crosslinkable or non-crosslinkable adhesion promoter to increase the adhesion between the sheet-like structure and a desired coating or printing material.
2) Prior Art
In the case of sheet-like moldings of polymeric materials, for example, films or sheets whose surfaces are generally nonhydrophilic, inadequate adhesive properties during further processing of the polymeric materials by coating or printing lead to undesirable problems. These include partial detachment of the coating from the sheet-like structure and the poor printability of the sheet-like structure.
Oriented films of plastics, in particular biaxially oriented films of polyethylene terephthalate, are used in large amounts as packaging and labeling material for various products, including food.
Since surfaces of films of plastic are generally hydrophobic, they are difficult to coat or print. For many packaging and labeling purposes, however, the films should be capable of accepting normal flexographic and/or gravure printing inks.
In order to remedy this deficiency, special inks which adhere directly to the surface of the films have been developed. However, many of these inks have formulations based on organic solvents, resin-like binders, and other components which are expensive and difficult to process.
Another possibility for improving the printability of plastic films is to apply a primer layer which both adheres to the film and accepts printing inks. Such primer layers consist, for example, of polymer materials based on vinylidene chloride terpolymers, as described in U.S. Pat. No. 2,698,240, or of water-dispersible copolyester primers, as disclosed in U.S. Pat. No. 4,252,885.
It is known that small amounts of sodium alkylsulfates can be added to aqueous dispersions of resin-like substances and these dispersions then applied as primer layers to biaxially oriented polyester films. The sulfates serve as emulsifiers or wetting agents for the intended uses described in, for example, U.S. Pat. No. 2,627,088.
However, none of these prior publications is concerned with the problem of improving the adhesion of plastic films or with solving this problem by the procedure described here.